Surface contamination detection using a radioactive labeled compound



United States Patent 3,297,874 SURFACE CONTAMINATION DETECTHGN USING A RADEQACTHVE LABELED COMPGUND John Lynde Anderson, Orlando, Fla, assignor to (Ileanometer Corporation, Wilmington, Del. No Drawing. Filed Dec. 21, 1961, Ser. No. 161,246 14 Claims. ((11. 250--1li6) This invention relates to a new method of detecting contamination on surfaces and more particularly to the detection of non-volatile organic contamination on metal and other surfaces.

This case is a continuation-in-part of my application Ser. No. 78,284, filed on December 27, 1960, now abandoned.

The rapid and quantitative detection of non-volatile organic contaminants either on metal or other surfaces or within spaces confined by such surfaces, particularly contaminants which are present only in very small amounts, is not practicable using existing methods and techniques. Thus, a method in general use involves the process of thoroughly wetting a metal surface with a solvent for the contaminant, of recovering the solvent quantitatively and then determining the amount of contaminant dissolved in the solvent. This method, however, determines only material which is removed from the surface and does not determine material which is left on, or is adjacent to, the surface after the test is completed. While it is known that the cleanliness of a surface can be determined by use of radioactive materials, the prior art methods, as, for example, that of Dvorkovitz et al. described in US. Pat. 2,968,733, are not adequate and not always operative, for the problem of contamination of metal surfaces often is a most serious one, particularly when the contamination may cause explosions or rapid, uncontrolled chemical reactions when in contact with other material for which the surface in its various forms is designed. For example, in handling liquid oxygen it is essential that the amount of organic contamination on the metal surfaces over which the liquid oxygen is standing or through which it is transferred must be kept to the irreducible minimum and must be quantitatively determined prior to permitting the liquid oxygen to come into contact with the metal surfaces involved, in order to insure maximum stability and safety. Testing procedures which introduce or leave contaminants behind must be avoided.

It is, therefore, an object of this invention to provide a new and useful method for the detection of contamination on surfaces. Another object is the provision of a method for the detection of non-volatile organic contamination on metal surfaces or within spaces confined by metal surfaces by a simple and economical method. A still further object is the provision of a method by which non-volatile hydrocarbon contamination on metal surfaces may be detected. Other objects will appear as the description of the invention proceeds.

These and other objects are accomplished in the present invention by applying a volatile radioactive labeled compound to a surface, by partially evaporating the radioactive labeled compound and at the same time evaporating substantially all of the solvent, if any, used to apply the labeled material, and by detecting the amount of radioactivity which is left on the surface being examined, the amount of remaining radioactivity after such evaporation retained on a previously scrupulously clean surface similarly treated being always less than that which is left on a contaminated similar surface and therefore the ratio of the amount of radioactivity on the contaminated surface to the amount on the previously scrupulously clean surface being a direct measure of the amount of contamination.

Another embodiment of my invention is accomplished by applying a volatile radioactive compound into the space enclosed by a surface, by partially evaporating the radioactive compound and at the same time substantially all of the solvent, if any, used to apply the labeled material, and by detecting the amount of radioactivity which is left retained within the space enclosed by the surface, the amount of radioactivity remaining after such evaporation from within the previously scrupulously cleaned space enclosed by a surface being less than that which is left within a contaminated space enclosed by a surface and therefore the ratio of the amount of radioactivity within the contaminated space enclosed by the surface to the amount within the previously scrupulously cleaned space enclosed by the surface being a measure of the amount of contamination.

More particularly my invention includes the process of applying a volatile radioactive labeled compound, preferably chemically compatible with the organic materials which are normally found on metal or similar surfaces or within spaces confined by such surfaces, .to such a surface or to within spaces confined by such surfaces, of partially evaporating the radioactive compound after such application only to the extent that the radioactive compound would substantially completely exaporate from a previously scrupulously clean surface or from within previously scrupulously clean spaces enclosed by a surface, of determining the residual radioactivity, and thus of detecting the contaminant.

A preferred embodiment of my invention is achieved by applying a volatile radioactive labeled compound to the surface, by partially evaporating the radioactive labeled compound and at the same time evaporating substantially all of the solvent, if any, used to apply the. labeled material, and by detecting the amount of radioactivity Which is left on the surface being examined and therefore a measure of the amount of contamination.

This invention will be further understood by reference to the description and to the examples given below which are given for purposes of illustration only and are not to be taken as limitative. In the examples, any parts or percentages given are by weight unless otherwise specified.

Example 1 Ten parts of a solution of 2000 parts of trichloroethylene and 1 part of l-decylbromide-l-C containing a total of 20 microcuries of radioactivity is added to a stainless steel surface containing dispersed uniformly on the surface 20 micrograms of non-volatile hydrocarbon per square inch of surface area. A similar amount of the radiochemical solution is added to a scrupulously clean stainless steel surface of similar area. Evaporation of each solution-wetted area is carried out for a period of forty-five seconds with a slow stream of dry gaseous nitrogen. Detection of the residual radioactivity on each surface demonstrates that the originally contaminated surface exhibits twice the activity of that shown by the originally scrupulously clean surface.

Example 11 One thousand parts of tetrachloroethylene-G containing 50 microcuries of radioactivity dispersed as liquid droplets in dry gaseous nitrogen is passed slowly through a 6 inch diameter scrupulously clean stainless steel tube containing a small amount of paper saturated with a non-volatile hydrocrabon wax. Following flushing of the entire stainless steel pipe with suflicient dry gaseous nitrogen to reduce the activity on the previosuly scrupulously clean surface so as to be virtually non-detectable over the background radiation, the paper saturated with hydrocarbon wax exhibits a significant and higher radioactivity level than does the adjacent metal surface.

3 Example III Three parts (0.1 milliliter) of a solution consisting of 8 milligrams of decyl bromide-l-C of specific activity 0.62 mc./mM. (millicuries per millimole) in milliliters of methylene chloride is added to the surface of a scrupulously clean glass plate and the rate of evaporation measured by recording the disappearance of radioactivity from the surface. As the solvent methylene chloride evaporates the counts per minute increases to a maximum and then rather sharply falls off and rapidly approaches the normal background when the radioactivity is measured by means of a thin window Geiger counter positioned approximately one half inch above the surface of the glass. The evaporation is controlled in part in this experiment by flowing over the surface of the glass plate a steady stream of clean dry nitrogen gas. The radioactivity as measured by the Geiger counter is recorded in counts per minute on a strip chart recorder. In this experiment background levels of radiation are reached in approximately 70 seconds. When an artificially contaminated similar glass plate to which 100 micrograms of crankcase grease has been added is treated similarly and the rate of evaporation observed under identical conditions of temperature and nitrogen gas flow, the rate of evaporation markedly decreases and in seventy seconds the radiation levels have decreased from a high of 700 counts per minute to approximately 200 counts per minute.

When a glass plate is contaminated with 250 milligrams of crankcase grease and all other conditions are held constant, the radiation levels decrease by only approximately 50 percent over the same time period.

Similar though not identical results are obtained when the surface under test is aluminum, polypropylene, nylon, and stainless steel.

Example IV When the surface being examined in Example III is an interior surface of a steel tank and the rate of nitrogen gas flow is markedly increased, the evaporation from a scrupulously clean surface is now substantially complete in seconds and evaporation from a contaminated surface (100 milligrams of grease) is not substantially complete after sixty seconds.

Example V One hundred lambda (0.1 milliliter) of a low boiling petroleum ether solution containing 2 microcuries of nonanol-1-C (specific activity of 2.0 mc./mM. per milliliter of solution) is added to a scrupulously clean glass plate and a stream of clean dry nitrogen gas is then directed onto the surface. The radioactivity in counts per minute measured one half inch above the surface by a Geiger- Mueller thin window tube ratemeter is recorded on a strip chart. Following evaporation of the solvent the residual nonanol-l-C evaporated substantially to background (less than counts per minute) within 30 seconds from a high of 1000 counts per minute.

When this experiment is repeated using a glass plate on which has been deposited 80 micrograms of crankcase grease the evaporative curve (as measured by the disappearance of radioactivity) shows a maximum of 1200 counts per minute and a reduction to 250 counts per minute after 50 seconds.

The slope of the evaporative curve obtained by expressing counts per minute against time in seconds at ten seconds from maximum is approximately 40 for the scrupulously clean surface and only 20 for the contaminated surface.

Substitution of an aluminum surface for the glass plate does not materially affect the rate of evaporation under either condition.

Example Vl In an experiment similar to Example V, the contamination introduced is 4 milligrams of shredded filter paper.

Under similar conditions of temperature and gas flow, the evaporation rate is intermediate and decreases from a maximum of 1250 counts per minute to 200 counts per minute in 70 seconds.

Example VII Dry air at 50 C. is led slowly from the bottom up through a vertical glass tube two meters in length which tube contains a tetrachloroethylene solution in which has been dissolved three millicuries of l,1,2,2-tetrabromoethane-Br (specific activity of 3.0 mc./mM.) and held at 50 C. The resultant saturated gas containing as vapor both tetrachloroethylene and 1,1,2,2-tetrabromoethane-Br is then led into a scrupulously clean 3 inch stainless steel pipe and valve assembly also held at 50 C. and into which has been introduced mg. of ma chine oil at one flange position joining the pipe and a' valve. Following detection of radioactivity in substantially the same concentration per unit volume of effiuent gas as is observed in the inlet gas the flow is stopped and the pipe and valve assembly isolated. The assembly is then permitted to cool to ambient temperature, that is to about 30 C. Measured externally with a scintillation counter, radiation is detected along the length of the pipe and valve assembly and a somewhat greater amount of radiation is detected adjacent to the contamination. Dry purge air is then metered slowly into the pipe and valve assembly and the rate of evaporation of the condensed 1,1,2,2-tetrabromoethane-Br into the purge gas is measured all along the pipe and valve assembly as the front of the purge gas progresses along the length. The rate of evaporation into the purge gas is observed to be similar throughout the pipe and valve assembly except in the area of the machine oil contamination where the rate is significantly lower.

Somewhat similar results are obtained when the evap oration is caused by evacuating the pipe and valve assembly and causing the condensed liquids to vaporize.

When these experimental conditions are observed in similar assemblies of glass, monel, aluminum, and linear polyethylene, similar evaporative results are obtained. Small effects are sometimes observed for changing surface conditions and surface materials in that the rate of evaporation from the clean surfaces vary slightly but in each case the rate of evaporation is condsidcrably lowered in the adjacency to the contamination.

When the experimental conditions are repeated except that the contaminant is (1) cod liver oil and (2) Ivory soap, similar results are obtained.

Example VIII In an experiment similar to Example VII the location of the contaminant is likewise confirmed when the solvent is butanol and the radioactive material iodoacetic acid-I and the contaminant 500 milligrams of proteinaceous residue.

Example IX In an experiment similar in radioactive material, solvent, temperature, surface, and nitrogen purge gas as in Example V, the presence of 10 milligrams of carbon black on a machined nylon surface is readily demonstrated in that the rate of evaporation is very much slower than from a clean nylon surface.

Example X An aerosol package contains perfiuorocyclobutane as propellant, 10.0 milliliters of methylene chloride and 50 microcuries of l,1,2,Z-tetraabromoethane-C (specific activity 3.5 mc./mM). When approximately one tenth microcurie per sq. in. is sprayed onto a clean machined brass surface of 10 square inch area and the solvent is evaporated under a steady stream of air at 30 C., the residual 1,1,2,2-tetrabromoethane-C disappears at a rate which indicates virtually complete evaporation in a period of one hundred seconds, the radiation being detected with a thin window Geiger counter mounted one half inch from the brass surface.

When the brass surface is contaminated with cutting oil at a level of milligrams per square inch and the aerosol application repeated and dry air is blown over the surface at the same rate, the rate of evaporation is slowed markedly and easily detectable residues of l,l,2,2-tetrabromoethane-C are observed even after 250 seconds.

Example XI Results somewhat similar to those obtained in Example X are observed where the aerosol package contains only perfiuorocyclobutane and 1,1,2,2-tetrabromoethane- C that is, the methylene chloride is eliminated, showing that additional solvents are not essential.

In still another experiment a mixture of decyl bromide- 1-C and nonanol-l-C is used as the radioactive element in order to effect preferential pick-up by contaminants varying in chemical composition. It is noted that this combination works well with contamination containing both carbohydrates and hydrocarbonaceous materials. Similarly, other mixtures of contaminants are treated with radioactive materials purposely compounded in mixtures to insure good pick-up by each individual contaminant in the composite impurity.

From the above it can be seen that in my invention a volatile radioactive labeled compound is used which preferably is compatible and miscible with the type of contaminant which it is desired to detect and to measure, and in general it is preferred that the radioactive labeled compound is chemically inert with both the surface and with the contaminant. In the event that the radioactive labeled compound selected is, in fact, reactive with the contaminant, it is necessary that the compound will be much less reactive with the surface itself so that after the evaporative phase of my invention, a difference in radioactivity will be observed. In addition, the labeled compound selected for a given surface or combination of surfaces will have a volatility which is normally less than the volatility of the particular solvent employed, if any. In any event the volatility of the radioactive labeled compound is such as to permit substantially complete evaporative conditions even when the surface is contaminated. Under these conditions the detection of residual radioactivity on the surface constitutes a positive demonstration that the contamination existed prior to the application of the test solution of the radiochemical.

The amount of radioactivity which is detectable is in general proportional to the amount of contamination which was originally present when the process of my invention is employed. The physical state and the chemical nature of the contamination also affect to some degree the amount of residual activity detected and the amount of radioactive labeled compound retained under the evaporative conditions. A small amount of contamination will cause the retention of only a small amount of residual activity whereas a large amount of chemically similar contamination will cause the retention of a relatively large amount of residual activity. The rate at which the solvent, if any, and the radioactive labeled compound are evaporated is not highly significant; however, the evaporative conditions are normally continued until substantially all of the solvent and the radioactive material would evaporate if no contaminant on the surface were originally present; that is, control conditions are met.

The type of contamination which is detected by my inventive process is that which is often found on metal surfaces. Such contamination may arise from cutting or threading oils, from greases and protective hydrocarbons, and in other ways. The particular radioactive labeled compound employed will determine in part the efficacy of the detection process and it is preferred that radioactive compounds be used which are compatible with hydrocarbons and other organic contaminants.

The particular radioactive labeled material employed in my invention is selected also for the type and energy of radiation emitted, for the chemical compatibility which the solvent used, if any, for the ability to be adsorbed on the surface of or absorbed within the contaminant and for such volatility as to enable substantially complete evaporation within a reasonable time from a previously scrupulously clean surface. The radioactive labeled compound is made radioactive by the presence of alpha, beta, or gamma emmitting radioisotopes. In general for measuring the radioactivity immediately adjacent to the contamination, compounds containing soft beta or gamma emitters are used while compounds containing hard beta or gamma emitters are required when the radioactivity is detected through a metal surface.

A preferred class of radioactive labeled compounds consists of organic compounds which are labeled with at least one of the group of beta emitters consisting of carbon -14, sulfur-35, phosphorous-32, chlorine-36 and hydrogen-3. Another preferred class of radioactive labeled compounds consists of organic derivatives containing one or more strong gamma emitter.

Some typical classes of volatile radioactive labeled compounds which are well suited for my invention are the hydrocarbons decane-C tridecane-C decalin-C isooctane-C and nonane-H halohydrocarbons such as decyl bromide-C tetrachloroethylene-C trichloroethylene-C hexachloropropene-C carbon tetrachloride C chloroform-C and dilluorotetrachloroethane-C ethers and thioethers such as ethylene glycol diethylether- C and dibutylthioether-S and phosphorous compounds such as tripropylphosphine labeled with phosphorous-32. In addition phenyl iodide-I paradichlorobenzene-C (or Cl), 1,1,2,2-tetrabromoethane-C (or Br 1,2- diiodoperfiuorocyclobutane-I 3,5-dichloro 1 iodobenzene-I 1,1,2-trichloro 2 bromoethane-Br naphthalene-C trimethyl phosphate-P phenyldichloro-phosphine-P l,1,4,4-tetrafiuoro 1,2,3,4 tetrabromobutane- Br, sulfuryl chloride-S iodine monochloride-I 1,1,2- trichloro-l-bromo-2-iodoethante-1 2,3 dibromo-1-pro panol-Br 3-hydroxypropionitrile-l-C iodoacetic acid- I methoxy acetic acid-C 1,3-propandiol-C, iodoacetic acid-C 1,1,2-tribromobutane-C ethyleneglycol- C glycol diacetate-C, nonanol-1-C and others are suitable. The above materials are used normally to detect material of organic nature, that is containing carbon and carbon skeletons. When it is of advantage to detect the presence of inorganic materials such as sand or rust, radioactive labeled materials which are largely inorganic or have marked affinity for the contaminant but not for the surfaces on which the contamination is located are selected such as: tetraethylsilicane-C, 3-iodosilicobenzoic acid-I omega-bromopropyltrichlorosilane- Br iron pentacarbonyl-Fe ferric chloride hexahydrate- Fe ferrocene-Fe and the like.

The radioactivity in my inventive process is detected by means of any suitable detector of radiation. For most purposes a Geiger-Muller tube and associated equipment is acceptable and convenient. Portable instruments used to detect radioactivity inside of long and complex metal tubes have had be designed for this purpose.

The surfaces examined by my inventive process may be almost any material used in commerce for construction of plates, pipes, valves, tanks, flanges, and the like. For example, metal, glass, fused ceramics, and plastic surfaces are all satisfactory and may be examined for the presence of contaminants. In general, the more porous the surface (that is, the greater the surface area) the less the differentiation between surface and contaminant with respect to retention of radioactive labeled material and therefore the less positive the absolute identification of contaminant. To illustrate, oil or grease is very readily detected on polished stainless steel but oil or grease on a surface prepared by compressing finely divided carbon black is more difficult to detect since the carbon black with a relatively high surface area absorbs the radioactive labeled material in much the same way as the contaminant absorbs it and the rate of evaporation from the compressed carbon black is nowhere near as rapid as from polished metal or glass. But by proper reference to the control the presence of the contaminant is detected as well as a fairly quantitative measurement of it. Knowing the nature of the surface, the operator of this invention will proceed accordingly.

It is apparent from this discussion that materials such as sand or finely divided iron oxide may be readily d tected on the surface of machined steel using my invention provided a radioactive labeled material is selected which is adsorbed more strongly by contaminants, such as iron oxide, than by the machined surface, since the evaporative rate from the iron oxide is less than that from the machined steel, other conditions, such as temperature and flow of gas over the surface being held constant.

Solvents such as tetrachloroethylene in Example VII and low boiling petroleum ether in Example V are not an essential part of this invention in its broadest aspects although in preferred embodiments solvents are used. Solvent functions are to increase the solubility, the rate of solution, and/or the adsorption or absorption of the radioactive labeled compounds on or in the contaminant to be detected. In addition the presence of the solvent vapor tends to increase the efficiency of the deposition on all surfaces, as is shown in Example VII. Many solvents may be used such as tetrachloroethylene, low boiling petroleum ether, butanol, perfiuorocyclobutane, methylene chloride, ethanol, acetone, formic acid, ethyl ether, and the like, Solvents with boiling points at least 100 C. less than the boiling points of the particular radioactive labeled material are used in order that appreciable residues of the radioactive labeled material are left over after substantially complete evaporation of the solvent.

In general, it is preferred to use a radioactive labeled material which has a boiling point at least 100 C. higher than the solvent boiling point in order to insure that, following the evaporation of the solvent, sufiicient radioactive labeled material is left behind for ready detection particularly in the case of previously scrupulously cleaned surfaces. Therefore, materials with boiling points in excess of 100 C. are preferred when low boiling solvents are employed and in excess of 130 C. when higher boiling solvents are employed. When no solvents are employed in my inventive process, the boiling point of the selected radioactive labeled material should be sufficiently high so that facile determination of the rate of evaporation is possible.

When two or more types of contaminants may be encountered, it is of advantage to use two or more particu lar radioactive labeled materials of such a nature that one is absorbed more strongly by a first type of contaminant and so forth. Thus, when attempting to locate and detect contaminants such as hydrocarbon grease and proteinaceous material, radioactive labeled materials such as 1,l,2,Z-tetrabromoethane-Br and iodoacetic acid-I may be used simultaneously in the same solvent or dispersant system. Also, the rate of evaporation of each may be determined independently of the other by means of suitable detection devices such as scintillation or solid state detectors coupled with channel analyzers so that the individual radiations of different energy levels are determined separately. This technique permits the detection of the location, the type, and the relative amount of each contaminant quickly and conveniently. Other counters which can be used include the Geiger counter and the proportional counter for beta and gamma rays. These various counters may be used externally or as probes moving within the enclosures being tested.

When using my inventive process, and particularly the aerosol method, many different types of propellants and/or solvents may be used. Thus, octafiuorocyclobutane, dibromodifiuoromethane, nitrogen, nitrous oxide,

butane, and the like may be used. Normally, it is desired to use non-flammable materials, such as the polyhalogenated materials. Included among these are monofiuorotrichloromethane, chloroform, carbon tetrachloride, dichloromethane, monochloromonofluoromethane, 1,1-difiuoropropane and 2,2-difluor-0propane, among others.

Since my inventive process depends on absorption or adsorption by the contaminant of the radioactive labeled material, it is important to insure that mixing of the radioactive labeled material with the contaminant be encouraged through use of the proper solvent, through providing a long enough time of contact prior to the encouragement of the evaporation, and by selecting the proper radioactive labeled material for the type of contaminant it is desired to detect.

In addition to the nature and amount of contaminant and the type of surface involved there are other factors which the operator of this invention will know or control in applying the principles of this invention. For example, the temperature and pressure affect the rate of evaporation and for greatest accuracy it is important that both be controlled closely. It is possible to carry out a series of reference studies so that in the event that either temperature or pressure cannot be controlled positively that reference to graphs or curves be made in order to correct for such variations. Detection systems such as Geiger counters, scintillation counters, and the like may be so constructed that temperature and pressure corrections are manually or automatically taken into account when the evaporative rate is determined. A further factor affecting the rate of evaporation is the amount, the velocity and type of gas flowing over the surface being examined. For highest accuracy the rate of gas fiow Will be accurately maintained so that evaporation from a scrupulously clean surface is always standardized as much as possible. Change of the rate of gas flow can be used to determine more accurately the amount of contamination under certain conditions and in general high rates of gas flow are used to cause the evaporation of relatively high boiling radioactive labeled materials while lower rates of gas flow are used with lower boiling materials. Proper shielding of the surface being examined is also important since extraneous gusts of wind or air will sometimes cause false indications of cleanliness. These factors are emphasized not by way of limitations on the broad applicability of this invention but to stress that proper controls or references be used in all measurements. While those skilled in the art will effect such controls, this emphasis may be a helpful reminder.

Although it is always possible to determine the rate of evaporation from a clean surface just prior to or just after the determination of the rate of evaporation from a contaminated surface, it is often of advantage to carry out the determination of the rate of evaporation from a scrupulously clean surface in a laboratory under more or less ideal situations and to use such a determination as a reference standard.

The sensitivity of the determination is a function of the amount of radioactive labeled material added in this inventive process. Thus, the higher the specific activity of the material, that is the amount of radioactivity per unit weight, the more sensitive is the test. Thus, if sufficient radioactivity for convenient detection is present in an amount as low as 1 microgram of material, dilution with 10 micrograms of contaminant will cause a very significant reduction in the rate of evaporation. However, if the same amount of radioactivity is present in micrograms of radioactive labeled material, the dilution by 10 micrograms of contaminant will not greatly affect the rate of evaporation until a very significiant amount of the radioactive labeled material has evaporated.

In general it is desirable to use specific activities in excess of 1.0 millicurie per millimole and preferably in excess of 3.0 millcuries per millimole. For example,

l,1,2,2 tetrabromoethane C at a specific activity of 3.5 mc./mM. has 0.01 microcurie in each microgram of material and this level is readily detectable. Thus, my inventive method provides a process for the detection of contaminants down to 1 microgram per square inch and even lower.

The processes of this invention are useful in many ways. They are used to detect the presence of hydrocarbon and other contaminants on the surfaces of many kinds and compositions, as for example, metal plates, valves, tanks and containers, pipes and tubes, and of other metal shapes. The residual hydrocarbon and other contaminants which are detected by the invention herein described are not in general determinable either qualitatively or quantitatively by prior art. The use of the invention herein described is especially important when the surfaces so examined are used for the storage or transfer of materials which react violently with the contaminants so detected since the process permits the determination not only of the level of contamination but it permits the establishment and determination of realistic levels of contamination to prevent the possibility of violent reaction.

While the invention has been disclosed herein in connection with certain embodiments and certain structural and procedural details, it is clear that changes, modifications or equivalents can be used by those skilled in the art; accordingly, such changes within the principles of this invention are intended to be included within the scope of the claims below.

I claim:

1. A process for detecting contamination on a surface which process comprises exposing the surface to be tested and any contamination thereon to a volatile radioactive labaled material; determining the rate of evaporation of said volatile radioactive material from the resultant exposed surface by measuring radioactive emissions using a radiation detector; and relating this rate of evaporation to the rate of evaporation obtained under control conditions in the absence of any contaminant.

2. A process in accordance with claim 1 in which a non-radioactive carrier gas is used along with the said labeled material.

3. A process in accordance with claim 1 in which the said material includes a non-radioactive solvent.

4. A process in accordance with claim 1 in which the said material includes a beta emitter.

5. A process in accordance with claim 4 in which said emitter is a material containing a C-14 atom.

6. A process for detecting contamination on a surface of a solid which process comprises exposing the surface to be tested and any contamination thereon to a volatile radioactive labeled material; determining the rate of evaporation of said volatile radioactive material from the resultant exposed surface by measuring the emission therefrom of radioactive waves using a counter; and relating this count to a control count obtained under like conditions but in the absence of any contaminant and to a control count obtained under like conditions but in the presence of a contaminant known both as to kind and amount.

7. A process in accordance with claim 6 in which the said surface 11S contained within an enclosure.

8. A process in accordance with claim 6 in which the said radioactive labeled material contains a radioactive compound which has a specific activity in excess of 1.0 millicuriies per millimole.

9. A process in accordance with claim 6 in which said radioactive material contains a mixture of radioactive compounds.

10. A process in accordance with claim 6 in which the said control counts are used as calibrations to measure the kind and the amount of contaminant present.

11 A process for the prevention of an explosion bebetween liquid oxygen and any contaminants present on surfaces to be contacted by said oxygen which comprises detectin g said contaminants by exposing the said surfaces and any contaminants thereon to a volatile radioactive material; determining the rate of evaporation of said volatile radioactive material from the resultant exposed surfaces; and determining whether this rate of evaporation is less than the rate of evaporation under control conditions in the presence of scrupulously clean surfaces, 21 lower rate indicating the presence of contaminants and the need for the removal thereof prior to said exposure of said surfaces to said oxygen.

12. A process for detecting contamination on a surface which process comprises exposing the surface to be tested and any contamination thereon to a volatile radioactive labeled material and thereby causing the retention of part of the labeled material by the contaminant; effecting the removal of. unreta-ined material by volatilization of the unretained material following said exposure; and measuring the radioactive emissions from the said surfaces following said volatiliization.

13. A process in accordance with claim 12 in which the said exposure is effected using a mixture of said radioactive material with a solvent for it which solvent is a non-radioactive material and has a boiling point at least about C. below that of said radioactive material.

14. A process for the detection of surface contamination which process comprises applying to the said surface and any contaminant thereon a material of low volatility; controlling the flow and amount of. gas over the surface and holding other factors affecting the evaporation under control; measuring the rate of evaporation of said material from said surface under said conditions; and comparing said rate to rates deter-mined similarly in the absence of contaminants and in the presence of contaminants known both as to kind and amount, thereby determining the amount of preexisting contamination, the said rate of evaporation being determined by using a radioactive labeled material as a said material of low volatility and by measuring its evaporation by means of a counter.

References Cited by the Examiner UNITED STATES PATENTS 2,520,058 8/1950 Reichertz 250-406 2,719,129 9/ 1955 Richardson 252305 2,957,989 10/1960 Hull 250-106 2,968,733 1/ 1961 Dvorkovitz 250-106 2,999,1162 9/1961 Fearon 250106 3,014,054 12/ 1961 Fries 250106 3,019,342 1/1962 Brooke 250-106 3,045,116 7/1962 Gaut 250106 3,102,101 8/ 1963 Hawley 2523 05 OTHER REFERENCES An article entitled Fighting Flux Contamination, Electronic Industries, vol. 10, No. 11, November 1959, pp. 110, 245, and 246.

Biological Applications of Tritium, by Thompson, Nucl-eonics, vol. 12, No. 9, September 1954, pp. 31 to 35.

Super Sleuths Trace Flow of Injected Gas, by Welge, The Oil and Gas Journal, August 29, 1955, pp. 77 to 79.

Using Traces in Refinery Control, by Hull, Nucleonics, vol. 13, No. 4, April 1955, pp. 18 to 21.

RALPH G. NILSON, Primary Examiner.

JAMES W. LAWRENCE, Examiner.

A. R. BORCHELT, Assistant Examiner. 

1. A PROCESS FOR DETECTING CONTAMINATION ON A SURFACE WHICH PROCESS COMPRISES EXPOSING THE SURFACE TO BE TESTED AND ANY CONTAMINATION THEREON TO A VOLATILE RADIOACTIVE LABELED MATERIAL; DETERMINING THE RATE OF EVAPORATION OF SAID VOLATILE RADIOACTIVE MATERIAL FROM THE RESULTANT EXPOSED SURFACE BY MEASURING RADIOACTIVE EMISSIONS USING A RADIATION DETECTOR; AND RELATING THIS RATE OF EVAPORATION TO THE RATE OF EVAPORATION OBTAINED UNDER CONTROL CONDITIONS IN THE ABSENCE OF ANY CONTAMINANT. 